Base stations in modern and next generation cellular communications networks transmit wideband signals. As such, a Power Amplifier (PA) of such a base station must be able to achieve wide, or broad, bandwidths that cover more than 10% relative frequency bandwidth (i.e., greater than 10% bandwidth relative to a center frequency of operation). Notably, relative frequency bandwidth is defined as Δf/fo, where Δf=(fH−fL), fo=[(fH+fL)/2], and fH and fL are an upper frequency and a lower frequency that define the outer edges of the bandwidth of operation. The most common high efficiency power amplifier architecture that is currently being implemented is the Doherty amplifier. As illustrated in FIG. 1, a conventional Doherty amplifier 10 includes a splitter 12, a class AB main amplifier 14, a class C or class B peaking amplifier 16, and a combining network 18 (sometimes referred to as a “Doherty combining network”) connected as shown. An efficiency enhancement of the Doherty amplifier 10 is implemented through load modulation of the main amplifier 14 from the peaking amplifier 16 at peak and back-off power levels. This load modulation is implemented through the combining network 18, which typically consists of two λ/4 (quarter wavelength) impedance transformers 20 and 22 with a high transformation ratio connected between the main and peaking amplifiers 14 and 16 and an external load 24 as shown. Due to the inherent band limiting characteristics of the λ/4 impedance transformers 20 and 22, the Doherty amplifier 10 tends to only support a narrow bandwidth (1-5% relative bandwidth) that is catered for a specific band of operation.
There are several existing approaches to design wider band impedance transformers that can be used in the combining network 18 of the Doherty amplifier 10. For instance, as taught in U.S. Pat. No. 7,602,241 and illustrated in FIG. 2 (which corresponds to FIG. 3B of U.S. Pat. No. 7,602,241), the bandwidth limitation of a single λ/4 impedance transformer can be overcome by using cascaded-connected impedance transformers that perform the same impedance conversion at each of N frequencies. However, the improvement in bandwidth carries a cost of substantial physical size (i.e., the cascade-connected impedance transformer is 2 to 5 times longer than a λ/4 impedance transformer). This increase in physical size is not compatible with current space limitations on Printed Circuit Boards (PCBs) in modern base stations.
In a similar manner, as taught in U.S. Pat. Nos. 8,193,857 and 8,339,201 and illustrated in FIG. 3 (which corresponds to FIG. 1 of U.S. Pat. No. 8,193,857), a gradual tapered impedance transformer 22″, such as a Klopfenstein impedance transformer, rather than an impedance transformer having multiple impedance steps, can be used to achieve wider bandwidth. However, the taper of the impedance transformer 22″ must be gradual and, as a result, the physical length of the impedance transformer 22″ would be considerably longer than a single λ/4 impedance transformer for a good tapered line. Therefore, again, the improvement in bandwidth carries a cost of substantial physical size. Also, due to the nature of the tapered structures, the size of the impedance transformer 22″ cannot be minimized by using common techniques such as bending.
To achieve multi-band performance, lumped element solutions have also been suggested as illustrated in FIGS. 4 and 5, where capacitors, circulators, varactor diodes, hybrids, and controllers are implemented as part of the impedance transformation process to achieve wider bandwidth. Specifically, as taught in U.S. Pat. No. 8,314,654 and illustrated in FIG. 4 (which corresponds to FIG. 1 of U.S. Pat. No. 8,314,654), the λ/4 impedance transformer 20 can be replaced by a tunable impedance inverter 26 that uses capacitors as a tuner and a circulator and is controlled by a digital controller 28. FIG. 5, which corresponds to FIG. 2 of U.S. Pat. No. 8,314,654, shows another implementation where the splitter 12 is implemented as a hybrid coupler, the tunable impedance inverter 26 is implemented as a hybrid coupler, and offset line circuits 30 and 32 are used to couple the main and peaking amplifiers 14 and 16 to the tunable impedance inverter 26 and the λ/4 impedance transformer 22, as illustrated. One problem with the lumped element solutions proposed in U.S. Pat. No. 8,314,654 is that these solutions require a number of components and an elaborate system to implement, which would result in a costly and complex solution.
As such, there is a need for a wideband impedance transformer for a combining network of a Doherty power amplifier that is physically small and has low-complexity.
Further, impedance transformers are a fundamental component in many Radio Frequency (RF) and microwave circuits. For instance, impedance transformers are used in functions such as, e.g., impedance matching, power splitting, and power combining. Conventional impedance transformers, such as λ/4 wave impedance transformers, have inherent band limiting characteristics where the typical relative bandwidths are 20% for a transformation ratio of 2. Due to the inherent band limiting characteristics of these λ/4 impedance transformers, impedance matching and power splitting/combining circuits tend to support only narrow bandwidths (i.e., 20% from central frequency of operation) and, as such, are catered for a specific band of operation.
There are several existing approaches to design wideband impedance transformers, but they become difficult to implement in a practical design. For instance, the bandwidth limitation of a single λ/4 impedance transformer can be overcome by using multi step transformers such as Chebyshev transformers as described in G. L. Matthaei, “Short-Step Chebyshev Impedance Transformers,” IEEE Transactions on Microwave Theory and Techniques, Vol. 14, No. 8, August 1966, pages 372-383 and U.S. Pat. No. 8,193,857 or cascaded-connected impedance transformers that perform the same impedance conversion at each of the N frequencies as described in U.S. Pat. No. 7,602,241, as illustrated above in FIG. 2. In this case, the improvement in bandwidth carries a cost of substantial physical size (2 to 5 times longer than a λ/4 impedance transformer), which is not compatible with current space limitations in many applications (e.g., a PCB of a base station transmitter). As discussed above with respect to FIG. 3, it is also possible to use gradual tapered impedance transformers rather than multiple impedance steps, but again the taper must be gradual, therefore the physical length would be considerably long for a good tapered line. Therefore, again, the improvement in bandwidth carries a cost of substantial physical size.
There have been studies where the length of the tapered impedance transformers have been attempted to be minimized by using common techniques such as meandering as illustrated in FIG. 6, which corresponds to FIG. 7 in A. Nesic et al., “A New Small Size Wideband Impedance Transformer,” 7th International Conference on Telecommunications in Modern Satellite, Cable and Broadcasting Services, Vol. 1, Sep. 28-30, 2005, pages A163-A166. However, the size of the overall wideband circuit still remains too large to be implemented on a practical design space of, e.g., a PCB.
To achieve multiband performance for an impedance transformer of a splitter/combiner, a unique multi-section structure illustrated in FIG. 7 with a slotted and arc-shaped solution has also been suggested in U.S. Pat. No. 4,490,695. In this case, the implementation of the parallel arc-shaped slots and the wedge shape of the design makes it complex to be implemented on a large scale design and posts constraints on the implementation on a specified board space area. Coupled line impedance transformers as illustrated in FIG. 8 and described in Ang, K. S. et al., “A Broad-Band Quarter-Wavelength Impedance Transformer With Three Reflection Zeros Within Passband,” IEEE Transactions on Microwave Theory and Techniques, Vol. 52, No. 12, December 2004, pages 2640-2644 and Xiang, Z. et al., “Design of Broadband Impedance Transformer Using Coupled Microstrip Transmission Lines,” 3rd IEEE International Symposium on Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, Oct. 27-29, 2009, pages 994-997 are able to provide wideband operation, but the size of the circuit posts the same real estate constraints as the other wideband structures. For the coupled line impedance transformer design, a λ/4 impedance transformer is coupled to another line which is connected through another λ/4 line to one side of the impedance transformer. In this manner, three λ/4 lines are folded together.
In all of the cases above, the wideband impedance transformer structures are large and, in many cases, complex. As such, there is a need for a wideband impedance transformer that is physically small and has low-complexity.